The ability of metallic materials to resist media erosion in corrosive media is called corrosion resistance of metals. High corrosion resistance in pure metals usually has one of the following three conditions:
1 Highly thermodynamically stable metal. It can usually be judged by its standard electrode potential, and its value is more stable than the positive one; the lower one is less stable. Precious metals with good corrosion resistance, such as Pt, Au, Ag, Cu, etc. fall into this category.
2 metal that is easy to passivate. Many metals can form a dense oxide film with a protective effect in an oxidizing medium. This phenomenon is called passivation. Among the metals that are most easily passivated are Ti, Zr, Ta, Nb, Cr, and Al.
3 The surface can form a metal that is insoluble and protects the corrosion product film. This is only the case when the metal is in a specific corrosive medium, for example, Pb and Al in H2SO4 solution, Fe in H3PO4 solution, Mo in hydrochloric acid and Zn in the atmosphere.
Therefore, according to the above principle, a series of corrosion resistant alloys are obtained by alloying in the industry, and generally there are three corresponding methods:
(1) To improve the thermodynamic stability of a metal or alloy, that is, to add an alloying element having high thermodynamic stability to a metal or alloy which is not resistant to corrosion, so as to form a solid solution and increase the electrode potential of the alloy to enhance the corrosion resistance. For example, adding Au to Cu, adding Cu, Cr, or the like to Ni is such a form. However, this method of adding precious metals in large quantities is limited in the application of industrial structural materials.
2 Adding easily passivable alloying elements such as Cr, Ni, Mo, etc., can improve the corrosion resistance of the base metal. A chromium-based stainless steel can be obtained by adding an appropriate amount of Cr to the steel.
Experiments have shown that in stainless steel, the amount of Cr should generally be greater than 13% to resist. The higher the Cr content, the better the corrosion resistance. Such stainless steels have good corrosion resistance in oxidizing media, but have poor corrosion resistance in non-oxidizing media such as dilute sulfuric acid and hydrochloric acid. This is because the non-oxidizing acid does not easily form an oxide film on the alloy and also has a dissolution effect on the oxide film.
3 Adding an alloying element that promotes the formation of a dense corrosion product protective film on the surface of the alloy is another way to obtain a corrosion resistant alloy. For example, steel is resistant to atmospheric corrosion due to the formation of a structurally dense compound of iron oxyhydroxide [FeOx·(OH)23-2x] which acts as a protective agent. The addition of Cu and P or P and Cr to the steel promotes the formation of such a protective film, whereby Cu, P or P, Cr can be used to form a low-alloy steel resistant to atmospheric corrosion.
Metal corrosion is the most dangerous spontaneous process in the industry. Therefore, the development and application of corrosion resistant alloys have great social and economic value.